Steps to reproduce:

1. Create a new URP project
2. Create New Material in the project
3. Create This Material's variant by right clicking on New Material > Create > Rendering > Material Variant
4. Open Render Pipeline Converter window (Window > Rendering > Render Pipeline Converter)
5. Checkmark Material Upgrade/Material Shader Converter (Name depends on Unity version)
6. Press Initialize and then Convert Assets

Actual results: Error is thrown:

Trying to set shader on a Material Variant.
UnityEngine.GUIUtility:ProcessEvent (int,intptr,bool&)

Expected results: No Error is thrown

Reproducible with versions: 6000.4.0a2, 6000.3.0.b4, 6000.2.6f2, 6000.1.17f1, 6000.0.58f2, 2023.3.0b10, 2022.3.62f1

Not reproducible with versions:  -